In the context of offshore oil and gas wells, jack-up drilling systems employ a concentric series of casings incorporating casing hangers enabling each casing to be suspended from the next outer casing such that the combined weight of the series of casings is ultimately carried by the outermost casing, and the series of casings is suspended at about the level of the mudline. This allows the drilling rig to operate in deeper water, and to disconnect from the casings after drilling, to enable the rig to move to another drilling location.
The outermost two of the series of casings generally have nominal diameters of 30 inches and 20 inches respectively. The 30 inch casing is cemented into the sea bed. The 20 inch casing is cemented into the sea where it projects downwards below the bottom end of the 30 inch, and is cemented into the 30 inch casing from the bottom end of the 30 inch casing up to a certain level below the mudline. The annulus between the 30 inch and 20 inch casings must be thoroughly washed clean of cement from the desired cement level up to the top of the casing system, both to allow disconnection of the drilling rig and to leave a clean profile onto which production equipment can be installed.
In prior art annulus washout systems, the 20 inch running tool is manipulated to open washports in the 20 inch casing. This procedure requires the 20 inch casing below the running tool to be suspended on an internal shoulder on the 30 inch casing. For structural reasons or because casings are preferably driven to refusal (limit of sea bed penetration), such an internal shoulder on the 30 inch casing cannot always be provided. The 20 inch casing is then cemented in tension, and use is made of a cumbersome and inefficient system of washpipes to clean out the annulus between the 20 inch and 30 inch casings. Even when a shoulder can be provided on the 30 inch casing, the non-rigid nature of jack-up drilling rigs results in bending loads that hamper free movement of the 20 inch running tool, hinder its manipulation to open the washports, cause damage to seals, and waste expensive rig time.